1. Field of the Invention
The present invention relates to demodulator circuits of the type which demodulate an amplitude-modulated signal to produce an analog voltage signal, and more particularly the invention relates to a demodulator circuit whereby when the output AC voltage signal of a chopper amplifier is converted to a DC voltage signal, the AC component contained in the DC voltage signal is reduced.
2. Description of the Prior Art
A prior art demodulator circuit such as disclosed in U.S. Pat. No. 4,067,233 for converting the output AC voltage signal of a chopper amplifier to a DC voltage signal, is constructed as shown in FIG. 1 of the accompanying drawings. In the Figure, numeral 1 designates a chopper, A an analog signal input terminal of the chopper, B an input terminal for receiving chopper clocks which determine the chopping frequency of the analog signal, 2 an AC amplifier connected to the output terminal of the chopper 1 and comprising an operational amplifier 21, an input capacitor 22, an input resistor 23 and a feedback resistor 24, and C the output terminal of the AC amplifier 2. In the demodulator circuit, numeral 3 designates a rectifying diode of the polarity shown, which is connected to the output terminal C to detect the positive half-cycles of the output signal of the AC amplifier 2. Numerals 4 and 5 designate a resistor and a capacitor constituting a smoothing filter for smoothing the output signal of the diode 3. Symbol D designates an output terminal of the demodulator circuit, and R a load resistor connected to the output terminal D.
The prior art circuit constructed as described above has the following contradictory disadvantages. When the analog voltage signal is chopped by such chopper clocks as shown in (a) of FIG. 2, the AC voltage signal of the AC amplifier 2 takes the form of the output signal shown in (b) of FIG. 2. In response to the positive half-cycle of this output signal, the diode 3 is rendered conductive so that the capacitor 5 is charged with a time constant which is determined by the resistor 4 and the capacitor 5, whereas in response to the negative half-cycle of the output signal, the diode 3 is rendered nonconductive so that the charge stored in the capacitor 5 is discharged through the load register R. Consequently, there is a disadvantage that the AC component V.sub.R shown in (c) of FIG. 2 remains as a ripple at the output terminal D, thus failing to effect demodulation with a high degree of accuracy. As a result, to overcome this disadvantage, if the value of the load resistor R is increased to increase the discharge time constant, there is the following disadvantage. When the peak voltage of the AC voltage signal from the AC amplifier 2 becomes lower than the terminal voltage of the capacitor 5, the diode 3 is rendered nonconductive so that the charge stored in the capacitor 5 is discharged with a time constant determined by the resistor R and the capacitor 5. However, due to the increased time constant, the discharge cannot satisfactorily follow up the output of the AC amplifier 2 thus greatly deteriorating the response.